FIG. 9 is a block diagram that illustrates the principle of displaying four sub-frame images on an NTSC TV system according to the related background art of this invention. The method for reducing the size of the four sub-frame images picked up with four TV cameras and properly aligning them on a single TV monitor will be described, referring to FIG. 9.
In FIG. 9, the images of numerals "1" to "4" are picked up with four TV cameras (image pickup devices) and divisionally displayed on a single TV monitor at the same time, by way of example. Specifically, data of the image "1" picked up with an image pickup device 9-1a are amplified by an amplifier 9-2a and thereafter output as an image 9-3a matching the NTSC system. The output image 9-3a is reduced in size in the manner to be described later to obtain image data. The image data are subjected to A/D conversion by an A/D converter 9-5a and stored in a RAM 9-6a under control of a controller 9-4. The image data stored in RAM 9-6a are read out therefrom under control of the controller 9-4 and subjected to D/A conversion at a D/A converter. After this D/A conversion, the sampled and reduced-in-size image 9-8a is obtained.
The above operations are also applied to the images picked up with the other image pickup devices 9-1b to 9-1d, and so the description therefor is not repeated, and similar reference numbers are used (a being replaced with b to d) for identical components.
The four reduced images 9-8a to 9-8d obtained as above are subjected to position alignment to be later described in detail and displayed as the image 9-10 of four sub-frame images 9-9a to 9-9d on the monitor screen of an NTSC system monochrome TV monitor.
Next, the method of reducing the size of the four images and displaying them on the monitor screen at predetermined locations will be described.
An image on a TV monitor is displayed by scanning the electron beam at a constant speed. Therefore, in order to display a size-reduced sub-frame image, the time axis should be changed beforehand. For such time axis changes, it becomes necessary to provide a storage device for temporarily storing the amount of information corresponding to one frame. The information is written to the storage at a slow sampling rate, and sequentially read out therefrom at a speed higher than the sampling rate.
Such an operation will be described with reference to FIGS. 8a and 8b. FIG. 8a illustrates the principle of reducing the size of an image by 1/2 in the vertical direction. As shown in FIG. 8a, to reduce the vertical direction by 1/2, the scan lines 8-2a, 8-2b of an image 8-1 picked up with an image pickup device are thinned every second scan line. The scan lines 8-2a are sampled, whereas the scan lines 8-2b are thinned or removed. Data on the sampled scan lines 8-2a are temporarily stored in a storage device (RAM) 8-3.
Next, the data are read out sequentially from RAM 8-3 at an ordinary readout speed so that an image 8-4 having a shorter time axis length in the vertical direction shown in FIG. 8a can be obtained.
FIG. 8b shows the principle of reducing the size of an image by 1/2 in both the vertical and horizontal directions. As shown in FIG. 8b, in order to reduce the size of an image by 1/2 in both the vertical and horizontal directions, the scan lines 8-2a and 8-2b of an image 8-1 picked up with an image pickup device are sampled while thinning every second scan line, in a similar manner to the case shown in FIG. 8a. Not all pixels on the sampled scan lines 8-2a are sampled, but the pixels are thinned every second pixel. The pixels 8-6a are sampled, whereas the pixels 8-6b are thinned or removed. The sampled pixels 8-6a are stored in RAM 8-3. Next, the sampled data are read out from RAM 8-3 at the ordinary readout speed. As a result, an image (sub-frame image) 8-7 with its size reduced by 1/2 both in the vertical and horizontal directions, i.e., with 1/4 of the original area, is obtained as shown in FIG. 8b.
The reduced image 8-7 thus obtained corresponds to each of the reduced images 9-8a to 9-8b shown in FIG. 9. The reduced images 9-8a to 9-8b are displayed on the monitor screen shown in FIG. 9 in the following manner. Namely, at the time when the scan line of the monitor screen comes to the position where the reduced-size image 9-9a is to be displayed, the data of the reduced image 9-8a stored in RAM 9-6a are read out and displayed. Similarly, at the time when the scan line of the monitor screen comes to the positions where the reduced-size images 9-9b to 9-9d are to be displayed, the data of the reduced-size images 9-9b to 9-9d are read out and displayed. In the above manner, the image 9-10 having four sub-frame images 9-9a to 9-9d can be obtained as shown in FIG. 9.
The above technique has the following problems.
(1) The horizontal resolution lowers by (resolution of an image pickup device)/(number of sub-frames in the horizontal direction). For example, in the case of the above-described four sub-frames, pixels in the horizontal lines are sampled while every second pixel is thinned. Therefore, the horizontal resolution is lowered by 1/2. PA1 (2) A/D converters and D/A converters are used for image data processing. These circuit components are required to be operable in the high frequency range. Therefore, highly sophisticated technology is required. In addition, the circuit becomes bulky which makes it impossible to miniaturize the overall system. Furthermore, such circuit components are very expensive, and result in the high cost of the overall system.